WASHINGTON (AP) -- Tinkering again with enforcement of the No Child Left Behind education law, the government plans to let some states fundamentally change how they measure yearly student progress.In an experiment that's been months in the making, up to 10 states will be allowed to measure not just how students are performing, but how that performance is changing over time.Currently, schools are judged based only on how today's students compare to last year's students in math and reading -- such as fourth-graders in 2005 versus fourth-graders in 2004.Many state leaders don't like the current system of comparing two different years of kids because it doesn't recognize changes in the population or growth by individual students.Education Secretary Margaret Spellings was announcing the "growth model" policy on Friday to a gathering of state school chiefs in Richmond, Virginia, The Associated Press learned."We're open to new ideas, but we're not taking our eye off the ball," Spellings said in remarks prepared for delivery to the state school officials.The latest shift in enforcement of the President Bush's No Child Left Behind law is significant politically. Frustrated states have been pleading for permission to measure growth by students, which may make it easier for schools to meet their goals and avoid penalties.Spellings has promised to be flexible in enforcing the law, one that is central to Bush's domestic agenda and often faces criticism in statehouses and schoolhouses.Other recent changes have dealt with testing, teacher quality and students with disabilities. Yet student progress is the cornerstone of the law.How it is measured has big implications.New measurement standardsSchools that receive federal poverty aid but don't make "adequate yearly progress" for at least two years face mounting penalties, from allowing students to transfer and providing tutoring to poor children to eventual restructuring of the school and its staff.Spellings said it makes sense to give schools credit for progress that students make.The states that win approval for the new flexibility, however, must do more than show growth. They still will have to get all children up to par in reading and math by 2014, as the law requires, and show consistent gains along the way.The Education Department, eager to show it is not weakening the law, will require states to take many steps before they can qualify for the "growth" option.States must have data systems to track individual students, close achievement gaps between whites and minorities, and prove they have at least one year of baseline testing.The law requires yearly testing in grades three through eight and once in high school.The department has not chosen the 10 states that will be part of the experiment. In practical terms, many states won't qualify because they don't have the kind of data systems to track individual students across grades. And others may not find the change helpful.To start, states that gain approval to measure student growth will also be required to chart progress the old way, comparing this year's students with last year's. The Education Department wants to see that data to help determine whether charting growth is a fair, accurate measure.Patricia Sullivan, director of the independent Center on Education Policy, praised federal leaders for showing flexibility and clearly outlining what states must do to get it.A growth model could benefit not just struggling students but also gifted ones who may be challenged anew to show their own yearly progress, beyond the school's standard benchmark."This is clearly what states have been asking for," Sullivan said. "It makes a lot of sense to measure growth. It's so discouraging for teachers when students make tremendous gains but don't get the credit because they don't get all the way over the bar."Copyright 2005 The Associated Press. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.